QThread Freezes GUI

import sys
from PySide2.QtCore import QThread
from PySide2.QtWidgets import (QApplication, QWidget, QPushButton, QComboBox, QVBoxLayout)

class Task(QThread):
    def run(self):
        # some heavy tasks
        print('task started')
        k = 0
        for i in range(10000):
            for j in range(50000):
                k += 1
        print('task finished')

class Gui(QWidget):
    def __init__(self):
        super().__init__()
        layout = QVBoxLayout()
        button = QPushButton('click me')
        button.clicked.connect(self.do_task)
        combobox = QComboBox()
        combobox.addItems(['1', '2', '3', '4', '5'])
        layout.addWidget(button)
        layout.addWidget(combobox)
        self.setLayout(layout)

    def do_task(self):
        self.thread = Task()
        self.thread.start()

if __name__ == '__main__':
    app = QApplication(sys.argv)
    window = Gui()
    window.show()
    sys.exit(app.exec_())